feat: add HEIC/HEIF format support for input and output

Add bidirectional HEIC support using system libheif CLI tools (heif-enc/heif-dec)
for HEVC encoding/decoding, since Sharp's bundled libheif only supports AV1.

- HEIC input: all tools now accept iPhone HEIC photos via heif-dec pre-processing
- HEIC output: convert tool produces true HEIC (HEVC) via heif-enc
- Docker: adds libheif-examples package for heif-enc/heif-dec CLI tools
- Tests: full conversion matrix (7x7), unit tests, and Playwright e2e tests
- Docs: updated OpenAPI spec, image-engine docs, getting-started, llms-full.txt

apps/docs/public/llms-full.txt

@@ -432,7 +432,7 @@ angle (number, degrees)
 direction: horizontal or vertical
 
 ### convert
-format (jpeg/png/webp/avif/tiff/gif), quality (1-100)
+format (jpeg/png/webp/avif/tiff/gif/heic), quality (1-100)
 
 ### compress
 quality (1-100), format (optional override)
@@ -449,7 +449,7 @@ grayscale, sepia, invert — no parameters
 ### Color channels
 red (-100 to 100), green (-100 to 100), blue (-100 to 100)
 
-Supported input formats: JPEG, PNG, WebP, AVIF, TIFF, GIF, SVG, RAW (via libraw)
+Supported input formats: JPEG, PNG, WebP, AVIF, TIFF, GIF, HEIC/HEIF, SVG, RAW (via libraw)
 
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